1. Field of the Invention
The present invention relates to measurement methods, exposure methods and device manufacturing methods, and more specifically, to a measurement method of measuring optical characteristics of an optical system that generates a pattern image on a predetermined surface, an exposure method of performing exposure taking into consideration the optical characteristics measured by the measurement method, and a device manufacturing method making use of the exposure method.
2. Description of the Background Art
The integration of semiconductor devices (integrated circuits) and the like is getting higher year by year, and accordingly the much higher resolving power is being required for a projection exposure apparatus, such as a stepper, which is a manufacturing apparatus of semiconductor devices and the like. Further, along with the high resolving power, it is also important to form a targeted pattern on an object to be exposed such as a wafer precisely following the designed value, and to improve the overlay accuracy of patterns of next and subsequent layers with respect to a pattern that has been already formed on the object to be exposed. For this purpose, optical characteristics of a projection optical system needs to be improved, and as the premise, it is important to accurately measure and evaluate the optical characteristics (including image-forming characteristics) of the projection optical system.
Accurate measurement of the optical characteristics of the projection optical system, for example, accurate measurement of an image plane of a pattern can be performed based on the assumption that a best focus position at each evaluation point (measurement point) within a field of the projection optical system can accurately be measured.
As a conventional measurement method of the best focus position of the projection optical system, a so-called CD/Focus method is representatively known. In this method, a predetermined reticle pattern (e.g. a line-and-space pattern or the like) serves as a test pattern and the test pattern is transferred to a test wafer at a plurality of positions in an optical axis direction of the projection optical system. Then, a linewidth value of a resist image (an image of the transferred pattern) that is obtained by developing the test wafer is measured using the scanning electron microscope (SEM) or the like, and the best focus position is computed based on a relation between the linewidth value and the wafer position in the optical axis direction of the projection optical system (hereinafter, also referred to as a “focus position” as needed).
However, in the CD/Focus method described above, for example, in order to measure the linewidth value of the resist image by the SEM, the focusing of the SEM needs to be performed strictly, and therefore it takes a very long time to perform the measurement at one point and several hours to several tens of hours were necessary for performing the measurement at many points. Thus, the throughput until the measurement results can be obtained drastically decreases. In addiction to this situation, the higher level is required also for repeatability of measurement errors or measurement results and therefore it becomes difficult to cope with it by the conventional measurement method.
Besides, a so-called SMP focus measurement method that is disclosed in U.S. Pat. No. 4,908,656 and the like is also known, in which a resist image of a wedge-shaped mark is formed on a wafer at a plurality of focus positions, and the length of the resist image in the longitudinal direction (which is the amplification of the change in the linewidth value of the resist image due to the difference in the focus position) is measured using a mark detection system such as an alignment system and the measurement result is used. In this SMP focus measurement method, however, since the measurement is usually performed using a monochromatic light, the effect of interference differs depending on the difference in shape of a resist image, which leads to measurement error (dimension offset), and also the resolution of the current image-capturing instrument (such as a CCD camera) is still insufficient. Further, it was difficult to increase the number of evaluation points because the test pattern is large in size.
For these reasons, a proposal of a measurement method has been expected, in which the optical characteristics of the optical system can be measured with high throughput using a measurement system having a low resolution, for example, a measurement device such as an alignment sensor based on an image-forming method of an exposure apparatus.